In digital transmission systems like ISDN or XDSL, the same pair of wire carries transmit signals and receive signals between point A and point B. Unless special precautions are taken, the reflected signal of the transmitter is superimposed on the received signal. The reflected signal of the transmitter is known as echo. The received signal will be called the far end signal in the following. Echo signals superimposed on the far end signal give rise to two major problems:
a) The receive path input has to be designed for high input levels. The signal to noise ratio (S/N) is decreased as a result.
b) Digital linear echo compensators in conventional ISDN and XDSL transmission systems are provided after the analog receive path. Such linear echo compensators are not able to subtract nonlinear distortion components from the echo. Thus, the signal to distortion ratio (S/D) has to be kept very low. In order to overcome the above mentioned problems it has been suggested to provide a scaled impedance replica of the transmission path consisting of resistors, capacitors and inductors outside a transmitter/receiver chip. The impedance replica generates a signal which is nearly identical to the echo generated on the real transmission path (tx-path). The signal of the real transmission line contains the echo plus the far end signal. The far end signal can therefore be retrieved by subtracting the replica signal from the signal on the real transmission line. This procedure is known as echo attenuation. For a sufficient echo attenuation it is necessary to have an exact impedance replica. A transmission channel may be simulated with different loops, each loop requiring a different impedance replica. The impedance replica has to be adjustable in order to achieve sufficient echo attenuation for each loop. Prior art semiconductor chips have pins to add or remove some of the passive components in the replica network. These pins and the necessity of external passive components to be attached to the chip increase the complexity of the hardware as well as the difficulty of operation. A further disadvantage is that the above-described external impedance replica is only a compromise solution and provides sufficient echo attenuation only for a few test loops.
It is accordingly an object of the invention to provide a device for echo attenuation in a digital transmission system, which overcomes the above-mentioned disadvantages of the heretofore-known devices of this general type and which provides sufficient echo attenuation.
With the foregoing and other objects in view there is provided, in accordance with the invention, a device for echo attenuation in a digital transmission system using an impedance replica of a transmission path, comprising an impedance replica of a transmission path, the impedance replica including a terminating resistance replica, a transformer replica connected to the terminating resistance replica, and a transmission line replica connected to the transformer replica.
In accordance with a further feature of the invention, the impedance replica further comprises a bridged tap replica connected in the transmission line replica.
In accordance with another feature of the invention, the echo attenuation device further comprises a replica tip line and a replica ring line, and the bridged tap replica is connected in parallel between the replica tip line and the replica ring line.
In accordance with yet another feature of the invention, the bridged tap replica comprises a series circuit including an inductance, a capacitance and a resistance.
In accordance with a further feature of the invention, the inductance, the capacitance and the resistance are adjustable.
In accordance with another feature of the invention, the transformer replica comprises a primary winding resistance replica, a secondary winding resistance replica connected to the primary winding resistance replica, a transformer main inductance replica connected to the primary winding resistance replica and to the secondary winding resistance replica, and a transformer stray inductance replica connected to the secondary winding resistance replica.
In accordance with yet another feature of the invention, the transformer main inductance replica is a variable inductance replica.
In accordance with a further feature of the invention, the replica tip line and the replica ring line form a replica loop, wherein the transformer main inductance replica is connected in parallel between the replica tip line and the replica ring line, and wherein the primary winding resistance replica, the secondary winding resistance replica, and the transformer stray inductance replica are connected in series in the replica loop.
In view of the object of the invention there is also provided a receiver/transmitter device of a digital transmission system comprising an impedance replica having a terminating resistance replica, a transformer replica connected to the terminating resistance replica, and a transmission line replica connected to the transformer replica for generating a replica signal for echo attenuation, and a subtractor connected to the impedance replica for receiving the replica signal and a signal of a transmission line.
The pure echo signal is established by connecting the device for echo attenuation, i.e. the replica loop, to the tip and ring terminals of the receiver/transmitter device in front of the terminating resistors.
In a preferred embodiment, the transformer replica is an on-chip component of a receiver/transmitter chip.
In accordance with another preferred embodiment of the invention, the impedance replica comprises a bridged tap replica connected in the transmission line replica, the bridged tap replica being an on-chip component.
In accordance with yet another preferred embodiment of the invention, the terminating resistance replica and the transmission line replica are on-chip components of the receiver/transmitter chip.
Thus the values of the components of the bridged tap replica and the main transformer inductance replica, which are adjustable, can be set by programming the receiver/transmitter chip.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a scaled impedance replica for echo attenuation in digital transmission systems, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.